Influences of Modified Sm2O3 on Thermal Stability, Mechanical and Neutron Shielding Properties of Aminophenol Trifunctional Epoxy Resin

The requirements regarding the weight and capacity reduction of neutron shielding materials have become an urgent issue for advanced nuclear facilities and plants. An epoxy-based neutron shielding material with high-temperature stability and good neutron irradiation resistance was designed in this paper to solve the above issue. Aminophenol trifunctional epoxy resin (AFG-90H) was compounded with samarium oxide (Sm2O3) by means of an ultrasonic-assisted method and the compatibility of Sm2O3 with the AFG-90H matrix was improved by 3-aminopropyltriethoxysilane (APTES) surface modification. Fabricated Sm2O3-APTES/AFG-90H composites exhibited improved thermal stability, glass transition temperature and Young’s modulus with increased Sm2O3-APTES content. Neutronics calculation results show that the neutron permeability of 2 mm-thick 30 wt% Sm2O3-APTES/AFG-90H was 98.9% higher than that of the AFG-90H matrix under the irradiation of the thermal neutron source. The results show that the proper addition range of Sm2O3-APTES is between 20% and 25%. The Sm2O3-APTES/AFG-90H composite is a promising neutron shielding material for advanced nuclear system.


Introduction
Neutron radiation exists not only in nuclear reactors, but also in various industrial, aviation and radiotherapy fields [1][2][3][4]. Uncharged neutrons can easily penetrate human tissues and cause ionization, which seriously threatens human health. Moreover, neutron radiation can also have a fatal negative impact on the service lifetime of electronic components and the environment [5]. As a result, the R&D on radiation protection materials is widely performed in the nuclear-related fields, including neutron shielding materials.
Neutrons shielding processes include two steps: moderating and absorbing [6,7]. High-energy neutrons (>1 MeV) first undergo inelastic scattering with substances with high atomic numbers and then lose energy, or undergo multiple elastic scattering with materials rich in hydrogen and then reduce energy to thermal neutrons (~eV). Thermal neutrons are easily captured or absorbed by elements with high neutron absorption cross-sections [8]. As described above, traditional shielding materials are roughly divided into metal, concrete and polymer materials according to their compositions. At present, the commercial metalbased shielding materials mainly include boron-containing stainless steel. Divya et al. pointed out that the serious problem with boron-containing stainless steels is the formation of low-melting point eutectics, makes this class of steel susceptible to heat cracking [9]. Maurya et al. studied the influence of various processing parameters on the strength, hardness and strain to failure of B 4 C particle reinforced Al-alloy matrix composites. When the content of B 4 C exceeded 10%, the strength and strain of B 4 C/Al to failure decreased due to particle agglomeration [10]. In addition, the commonly used concrete shielding materials mainly include boron-containing concrete and cement composites. Chidiac et al. studied the heat evolution, compressive strength and neutron shielding capabilities of hydrated mixes containing 0, 25, 50 and 75 wt% of B 4 C. The results show that a concrete mix containing up to 50 wt% B 4 C was found to be beneficial to the development of concrete properties and to effective neutron shielding [11]. Sevim et al. studied the strength and fresh properties of borogypsum concrete. They showed that an addition of 4 wt% of boron sludge could reduce the 90-day compressive strength of concrete by 65 wt% [12]. However, the requirements regarding the above materials in nuclear power stations and facilities are a large engineering amount and a heavy weight [13]. There are limitations in transportation and small modular manufacturing. Currently, the fourth-generation advanced nuclear power system puts forward the target of "integration of structure and function" with the demand for weight and capacity reduction. Therefore, developing new neutron shielding materials with outstanding properties has become one of the promoted ways to satisfy the requirements of neutron protection.
Blending hydrogen-rich polymer matrix with high neutron cross-section absorbing elements can integrate neutron moderation and absorption. So, these materials are expected to become alternative materials for metals and concrete in nuclear facilities, such as polyethylene (PE) [14,15], high-density polyethylene (HDPE) [16,17], epoxy resin (EP) [18,19], ethylene propylene diene monomer (EPDM) [20] and polyimide (PI) [21,22], etc. Although PE and EPDM matrix have good processability, they have low thermal stability and it is difficult to use them at a temperature higher than 100 • C for a long time. PI has high temperature stability and good mechanical properties. Wu et al. studied composites based on carbon fiber-reinforced carborane-containing polyimides. The results show that the thermoset polyimide matrixes exhibited high thermal stability with T 5 > 800 • C both in nitrogen and air environments [23]. However, the synthesis process of PI is more complex than other polymers and it is mostly used in membrane products. It is difficult to break through to large-scale preparation. Among the above polymers, EP has good chemical stability and structure stability. Jiang et al. blended B 4 C particles with AFG-90H to develop a new type of AFG-90H epoxy resin-based composite, and the results show good neutron shielding and mechanical performance at high temperatures [24]. Moreover, owing to the aromatic ring in the main chain of EP, it is mechanically stable under long-term irradiation [25]. Therefore, a new type of three functional epoxy resin AFG-90H was synthesized as the matrix of a neutron shielding material. Compared with ordinary EP, it has better neutron irradiation resistance, mechanical stability and thermal stability [26].
In order to improve the thermal neutron absorption performance of EP, it is important to select suitable neutron absorbers. The average macro-absorption cross-sections (∑) of the neutron-absorbing elements Gd, B, Cd and Sm are 49,700, 3835, 2520 and 5922 barns at thermal neutron energy, respectively [27]. Figure 1 shows the absorption cross-sections of these four elements at different neutron energies. Gd has the highest thermal neutron capture cross-section among all of the stable nuclei. However, the neutron capture reaction for 157 Gd and 155 Gd emits a γ-ray cascade with a total energy of~8 MeV [28]. B and its compounds (e.g., B 4 C and BN) are good thermal neutron absorbers and are widely used in neutron shielding materials. Moreover, B compounds absorb neutrons through the reaction 10 B(n,α) 7 Li with the concomitant generation of helium bubbles [29]. Such helium bubbles cause the premature failure of materials. As is known, Cd is toxic, carcinogenic and harmful to human health and is not suitable to be used as a radiation protection element. It is usually used as an element in the Ag-In-Cd control rods of reactors [30].
1 List of small modifications 1. Figure 1 in Section 1 "Introduction" We have further beautified Figure 1 and unified the coordinate axis format. Please replace the Figure 1 in Section 1 with the following figure:

Figure 2 in Section 2 "Experimental"
We also further simplify and beautify Figure 2 in Section 2 to make it clearer. Please replace Figure 2 in Section 2 with the following figure: Samarium is found in many natural minerals, such as bastnaesite, monazite and samarskite [31]. The total reserve of Sm in the earth is approximately 2 million tons [32]. Sm is most commonly sold in the form of Sm 2 O 3 , which is one of the cheapest rare earth oxides on the market (USD~40/kg in China) [33]. Additionally, Sm is also a suitable neutron absorber; the natural abundance of 149 Sm is 13.9%, and most of its neutron capturing and decay products are also other isotopes of samarium [34]. Sm has a wider neutron absorption spectrum of 10 −7 -10 −5 MeV compared to Gd and B. Therefore, it has the potential to be used as a neutron absorber for application in shielding materials.
The high content of fillers can cause their uneven distribution in the polymer matrix and poor interface interaction. This defect reduces the mechanical stability of EP composites. One issue is that high neutron shielding and mechanical performances cannot be well balanced. Generally, this is due to large differences in physicochemical properties between the inorganic filler and the epoxy resin matrix [35]. Constructing a chemical bond "bridge" is the key to strengthening interfacial interactions. This "bridge" can enable the orderly grafting of inorganic particles and organic molecular chains to improve the dispersion of fillers in the resin matrix [36]. Covalent modification is a simple fabrication method for particles. The silane coupling agent APTES contains two different chemical functional groups. One end of the functional group chain can react with silanol groups and then connect to the surface of inorganic materials such as glass fiber, silicate and metal oxide, etc. The other end can form covalent bonds with resin molecular chains. Thus, the coupling of the two incompatible materials can realize the equivalent function of a "bridge".
In this work, Sm 2 O 3 /AFG-90H composites were prepared by solution casting and ultrasonic mixing of epoxy resin matrix (AFG-90H) and surface-modified Sm 2 O 3 . The Sm 2 O 3 was used as the neutron absorber. Synchronous thermal analysis (TG-DSC) was used to analyze the thermal stability and glass transition temperature of the materials. The integrated neutron transport and safety evaluation software SuperMC [37] was used to analyze the neutron shielding performance of the materials coupled with the data library HENDL1.0 [38]. The microstructure and Fourier-transform infrared (FT-IR) spectroscopy of the Sm 2 O 3 -APTES and Sm 2 O 3 -APTES/AFG-90H were analyzed. The composites were characterized by X-ray diffraction (XRD). The influence of the mass fraction of fillers in the matrix on the mechanical properties of materials was studied.

Surface Modification of Sm 2 O 3 Powder
The silane coupling agent (APTES) was employed to modify the interfacial interaction of Sm 2 O 3 particles. First, Sm 2 O 3 particles were dispersed in 60 mL of toluene and stirred under nitrogen gas environment for 20 min before drying. This process was undertaken to clean the surface of the Sm 2 O 3 particles. Secondly, Sm 2 O 3 particles were ultrasonicated in 50 mL absolute ethanol for 40 min to form hydroxyl groups on the surface of the Sm 2 O 3 particles. Absolute ethanol, deionized water and APTES were prepared as a solution with a volume ratio of 9:1:2.5. The glacial acetic acid was used to adjust the pH to 5. The hydrolyzed APTES solution was obtained after full stirring. Under mechanical stirring, the hydrolyzed APTES solution was added dropwise to the Sm 2 O 3 suspension to react at room temperature for 24 h. After centrifuging the suspension, the particles were cleaned with absolute ethanol and acetone successively. Finally, the clean Sm 2 O 3 powders were put into the oven and dried for 5 h. The surface modification procedure is shown in Figure 2a.

Fabrication of Sm 2 O 3 -APTES/AFG-90H
To prepare Sm 2 O 3 -APTES/AFG-90H composites, different contents of Sm 2 O 3 -APTES (0-30 wt%) were dispersed into ethanol and ultrasonicated to form a uniform suspension. Subsequently, DDS and AFG-90H were configured into a pre-cured resin at a weight ratio of 5.58:10 and stirred in an oil bath at 105 • C to a low-viscosity state. Then, the dried Sm 2 O 3 -APTES particles were poured into the resin solution under mechanical stirring and ultrasonically treated for 1 h to ensure that the Sm 2 O 3 -APTES was evenly dispersed in the resin matrix. The mixed resin solution was dried at 80 • C to remove ethanol. Finally, the stirred mixture was poured into a Teflon mold and degassed at 90 • C to remove air bubbles. The Sm 2 O 3 /AFG-90H composites was cured at 185 • C for 3 h and then removed from the mold. The diagram of the curing reaction is shown in Figure 2b.
List of small modifications 2 3. Figure 6 in Section 3 "Results and discussion" We modified "void" to "Voids" in Figure 6h. Please replace Figure 6 in Section 3 with the following figure: 4. Figure 10 in Section 3 "Results and discussion" We modified "void" to "Voids" in Figure 10f. Please replace Figure 10 in Section 3 with the following figure:

Characterization Methods
Fourier transform infrared (FT-IR) spectra were measured using a Nicolet iS 10 spectrometer with the KBr squash technique and the test device was used to characterize the peaks of functional groups. X-ray diffraction (XRD) analyzed on the AFG-90H and Sm 2 O 3 particles were performed on Rigaku Ultima IV X-ray diffractometer from 10 • to 80 • . Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed in an air atmosphere using a NETZSCH 449 F3 synchronous thermal analyzer under the heating rate of 10 • C/min. TG-DSC was used to analyze the thermal stability and glass transition temperature of Sm 2 O 3 -APTES/AFG-90H. Scanning electron microscopy (SEM, Carl Zeiss AG, Oberkochen, Germany) was performed to characterize the morphology of Sm 2 O 3 particles and the surface of Sm 2 O 3 /AFG-90H composites, including its tensile cross-section of the tensile fracture test. Static mechanical testing was performed in accordance with ASTM D638-91 using an Instron 3369 Universal Testing Machine with a loading rate of 2 mm/min. At least four separate dumbbell-shaped samples were tested for each experiment. The Young's modulus (E) and elongation at break (e) of all samples were tested via the Instron 3369 machine. The Shore hardness of specimens was measured using a Shore D durometer in accordance with ASTM D2240 Shore hardness test method. The Shore hardness was measured five times at the surface with an interval of at least 6 mm for each sample and the average values were calculated.

Surface Modification of Sm 2 O 3 Powders
The infrared spectra of APTES, Sm 2 O 3 and Sm 2 O 3 -APTES are shown in Figure [39]. The Si-O-Si bond formed on the surface of non-Si-based materials was a weak chemical interaction formed between two APTES molecules [40]. When comparing the FTIR spectrums of APTES and Sm 2 O 3 -APTES, no obvious peaks were observed in the original Sm 2 O 3 powders. In contrast, as shown in the FTIR spectrum of Sm 2 O 3 -APTES, all types of characteristic peaks of N-H at 1567 and 1488 cm −1 were more prominent than in the APTES spectrum. This indicated that more Sm 2 O 3 -APTES particles were connected to each other through N-H bonds (as shown in Figure 4). In addition, the weak N-N characteristic peak can be seen at 1372 cm −1 . The FT-IR results indicate that the silane groups in APTES successfully formed covalent bonding with the surface of Sm 2 O 3 . a loading rate of 2 mm/min. At least four separate dumbbell-shaped samples were tested for each experiment. The Young's modulus (E) and elongation at break (e) of all samples were tested via the Instron 3369 machine. The Shore hardness of specimens was measured using a Shore D durometer in accordance with ASTM D2240 Shore hardness test method. The Shore hardness was measured five times at the surface with an interval of at least 6 mm for each sample and the average values were calculated.

Surface Modification of Sm2O3 Powders
The infrared spectra of APTES, Sm2O3 and Sm2O3-APTES are shown in  [39]. The Si-O-Si bond formed on the surface of non-Si-based materials was a weak chemical interaction formed between two APTES molecules [40]. When comparing the FTIR spectrums of APTES and Sm2O3-APTES, no obvious peaks were observed in the original Sm2O3 powders. In contrast, as shown in the FTIR spectrum of Sm2O3-APTES, all types of characteristic peaks of N-H at 1567 and 1488 cm −1 were more prominent than in the APTES spectrum. This indicated that more Sm2O3-APTES particles were connected to each other through N-H bonds (as shown in Figure 4). In addition, the weak N-N characteristic peak can be seen at 1372 cm −1 . The FT-IR results indicate that the silane groups in APTES successfully formed covalent bonding with the surface of Sm2O3.

XRD Patterns of Sm2O3 and Sm2O3-APTES/AFG-90H Composites
The XRD diffraction patterns of Sm2O3, Sm2O3-APTES and Sm2O3-APTES/AFG-90H are shown in Figure 5. The multimodal diffraction pattern of Sm2O3 showed that it belonged to the monoclinic crystal system. In addition, the Sm2O3-APTES showed a similar

XRD Patterns of Sm 2 O 3 and Sm 2 O 3 -APTES/AFG-90H Composites
The XRD diffraction patterns of Sm 2 O 3 , Sm 2 O 3 -APTES and Sm 2 O 3 -APTES/AFG-90H are shown in Figure 5. The multimodal diffraction pattern of Sm 2 O 3 showed that it belonged to the monoclinic crystal system. In addition, the Sm

XRD Patterns of Sm2O3 and Sm2O3-APTES/AFG-90H Composites
The XRD diffraction patterns of Sm2O3, Sm2O3-APTES and Sm2O3-AP are shown in Figure 5. The multimodal diffraction pattern of Sm2O3 show longed to the monoclinic crystal system. In addition, the Sm2O3-APTES sho trend as the original Sm2O3, indicating that the crystal structures of Sm2O3 d during the surface modification process of APTES. Obviously, the existence teristic signal of Sm2O3 at 2θ = 25-65° in the XRD pattern of Sm2O3/AFG-90 confirmed the successful incorporation of Sm2O3-APTES into the epoxy res APTES/AFG-90H exhibits a broad hump at approximately 15°, indicating t properties of the matrix material [41].

Morphology Analysis
The morphology of the fractured surface of the Sm2O3-APTES/AFG-9 was shown in Figure 6. The particles show an irregular polyhedron shap protrusions and slight agglomeration (as shown in Figure 6a,b). The section 90H resin was smooth (as shown in Figure 6c,d). With the increase in Sm2O tent, the fractured surface changed from smooth to rough. When the Sm2O tent was less than 20 wt%, it was uniformly dispersed into the matrix resin a fewer interfacial voids (as shown in Figure 6e,f). At a content above 20 wt%

Morphology Analysis
The morphology of the fractured surface of the Sm 2 O 3 -APTES/AFG-90H composite was shown in Figure 6. The particles show an irregular polyhedron shape with fibrous protrusions and slight agglomeration (as shown in Figure 6a,b). The section of pure AFG-90H resin was smooth (as shown in Figure 6c,d). With the increase in Sm 2 O 3 -APTES content, the fractured surface changed from smooth to rough. When the Sm 2 O 3 -APTES content was less than 20 wt%, it was uniformly dispersed into the matrix resin and there were fewer interfacial voids (as shown in Figure 6e,f). At a content above 20 wt%, the particles began to agglomerate (as shown in Figure 6g,h) due to the increase in the probability of the interconnection of active free groups on the particle surface. Because the interfacial energy of the formation of aggregates between particles was less than the binding energy of particles and the matrix molecular chain, there was no obvious separation of aggregates from the matrix, indicating that the adhesion with the resin matrix was good, because the free radicals on the surface of the filler helped to strengthen the interfacial bonding energy between Sm 2 O 3 -APTES and AFG-90H. However, when the Sm 2 O 3 -APTES content was 30 wt%, an increase in the number of pores in the matrix was observed. Therefore, the extension of dispersion for the Sm 2 O 3 -APTES particles in the matrix should be optimized during the preparation process. In addition, the particle size distribution is shown in Figure 7. The average size of Sm 2 O 3 -APTES was approximately 1.16 µm. Sm 2 O 3 -APTES particles of micrometer size can have a pinning effect on the epoxy resin matrix, which will be described in detail in Section 3.5.

Figure 6 in Section 3 "Results and discussion"
We modified "void" to "Voids" in Figure 6h. Please replace Figure 6 in Section 3 with the following figure: 4. Figure 10 in Section 3 "Results and discussion" We modified "void" to "Voids" in Figure 10f. Please replace Figure 10 in Section 3 with the following figure: between Sm2O3-APTES and AFG-90H. However, when the Sm2O3-APTES content wt%, an increase in the number of pores in the matrix was observed. Therefore, the sion of dispersion for the Sm2O3-APTES particles in the matrix should be optimize ing the preparation process. In addition, the particle size distribution is shown in 7. The average size of Sm2O3-APTES was approximately 1.16 μm. Sm2O3-APTES pa of micrometer size can have a pinning effect on the epoxy resin matrix, which described in detail in Section 3.5.

Thermal Performance
The TG, DTG and DSC thermal analysis curves of Sm2O3-APTES/AFG-90 shown in Figure 8. The mass loss temperatures of the TG curve are listed in Table 2, T5 represents the temperature with a mass loss of 5 wt%, so for T10, T50 and Tmax.

Thermal Performance
The TG, DTG and DSC thermal analysis curves of Sm 2 O 3 -APTES/AFG-90H are shown in Figure 8. The mass loss temperatures of the TG curve are listed in Table 2, where T 5 represents the temperature with a mass loss of 5 wt%, so for T 10 , T 50 and T max .  As shown in Figure 8a, the two-stage decomposition process was found in all specimens, and these stages showed the conformation of the thermal decomposition trend for AFG-90H matrix. In the first stage, the thermal weight loss occurred at approximately 350 °C and the weight loss step continued to 430 °C. This decomposition reaction is attributed to the decomposition reaction of the epoxy network [42,43]. Owing to the existence of rigid structures (i.e., a benzene ring and an epoxy group) in the resin matrix, further decomposition of the composite was hindered. So, the curve remained flat in the temperature range from 430 to 530 °C [44]. In the second stage, the thermo-gravimetric range was from 530 to 650 °C. The polymer chain was completely decomposed and then the Sm2O3 was entirely separated from the matrix. The mass loss temperature was listed in Table 2, T5 followed a downward trend when the Sm2O3-APTES content increased from 0 to 10 wt% and an upward trend when the content increased from 15 to 30 wt%, while the same was observed for T10. When the Sm2O3-APTES content was 30 wt%, T5 and T10 increased by 5.4% and 3.6%, respectively, compared with that of the pure AFG-90H matrix. Microparticles increased physical crosslinking points and enhanced interfacial interactions. They play a positive role in inhibiting the initial decomposition [45].  As shown in Figure 8a, the two-stage decomposition process was found in all specimens, and these stages showed the conformation of the thermal decomposition trend for AFG-90H matrix. In the first stage, the thermal weight loss occurred at approximately 350 • C and the weight loss step continued to 430 • C. This decomposition reaction is attributed to the decomposition reaction of the epoxy network [42,43]. Owing to the existence of rigid structures (i.e., a benzene ring and an epoxy group) in the resin matrix, further decomposition of the composite was hindered. So, the curve remained flat in the temperature range from 430 to 530 • C [44]. In the second stage, the thermo-gravimetric range was from 530 to 650 • C. The polymer chain was completely decomposed and then the Sm 2 O 3 was entirely separated from the matrix. The mass loss temperature was listed in Table 2, T 5 followed a downward trend when the Sm 2 O 3 -APTES content increased from 0 to 10 wt% and an upward trend when the content increased from 15 to 30 wt%, while the same was observed for T 10 . When the Sm 2 O 3 -APTES content was 30 wt%, T 5 and T 10 increased by 5.4% and 3.6%, respectively, compared with that of the pure AFG-90H matrix. Microparticles increased physical crosslinking points and enhanced interfacial interactions. They play a positive role in inhibiting the initial decomposition [45].
The above results show that the agglomeration occurring with a high content of Sm 2 O 3 -APTES had a strong blocking effect on the thermal movement of the AFG-90H molecular chain, such as the weak chemical interaction between Sm 2 O 3 -APTES and AFG-90H. However, the maximum decomposition temperature T max of Sm 2 O 3-APTES/AFG-90H decreased compared with that of pure AFG-90H. The decreased maximum decomposition temperature T 2 can also be seen in Figure 8b. In addition, T g at the same heating rate as TG was calculated through the DSC curve (as shown in Figure 8c). The T g of Sm 2 O 3 -APTES/AFG-90H with a low filler content (≤15 wt%) was irregular. This was due to the weak chemical force between the matrix molecular chains and Sm 2 O 3 -APTES with a low content. When the Sm 2 O 3 -APTES content was 30 wt%, T g increased by 3.16% compared with that of pure AFG-90H.
The trend of T g of Sm 2 O 3 -APTES/AFG-90H is shown in Figure 8d. Generally, T g of polymer composites is affected by molecular weight, cross-linking density and particlematrix interface area [46]. The trend of T g vs. temperature could be due to the combined effects of two opposite factors [47]. On the one hand, the strong interfacial interaction between AFG-90H matrix and Sm 2 O 3 -APTES fillers could partially limit the motions of polymer chain segments and delay the occurrence of relaxation behavior at the glass transition region. This will reduce the flexibility of epoxy chain at high temperature and then lead to an increase in T g . On the other hand, the addition of fillers could cause difficulty for the composites to achieve the same level of curing as the neat AFG-90H, and thus the cross-liking density decreases and leads to a decrease in T g . In this work, when the content of Sm 2 O 3 -APTES is more than 20 wt%, the free volume of the resin decreases and the thermal movement of the chain end is limited by entangled Sm 2 O 3 -APTES agglomeration. Similar results can be found in Refs. [48,49]. The polymer chains need to absorb more energy to overcome the energy barrier, which leads to an increase in T g . Conversely, T g changes irregularly due to the interaction of the above two factors. Ref. [50] showed similar results on the polymer incorporated with APTES-modified particles.

Mechanical Properties of Sm 2 O 3 -APTES/AFG-90H
The mechanical properties of Sm 2 O 3 -APTES/AFG-90H are shown in Figure 9, including Young's modulus, elongation at break, tensile strength and Shore hardness. It is clear that the Young's modulus of Sm 2 O 3 -APTES/AFG-90H composites increased with an increase in Sm 2 O 3 -APTES content (as shown in Figure 9a). When the content of Sm 2 O 3 -APTES was 0-5 wt%, the Young's modulus increased slowly due to the good interfacial bonding between Sm 2 O 3 -APTES and the AFG-90H chains. When its content was 10-20 wt%, the Young's modulus growth increased faster with uniform dispersion of Sm 2 O 3 -APTES and good interface compatibility, which synergistically improves the modulus of composite materials. When its content was 30 wt%, the increased probability of Sm 2 O 3 agglomeration could destroy the mechanical properties of the composite material, which was shown in Figure 9b. From the perspective of Shore hardness, the surface hardness of the material gradually increases with Sm 2 O 3 -APTES and finally reaches a saturation value (as shown in Figure 9c). The texture of the composite material is brittle and harder, indicating that Sm 2 O 3 -APTES can slightly increase the surface hardness of AFG-90H. Composite stiffness significantly depended on the Sm 2 O 3 -APTES particle content but not the particle/matrix adhesion, since the fillers had a much higher modulus than the matrix [51]. Polymers 2022, 14, x FOR PEER REVIEW 11 of 17 One criterion for judging the toughness of polymer materials is the elongation at break. The elongation at break of the composite reached a peak of 0.68%, and Tg was the lowest when the Sm2O3-APTES content was 15 wt% in this study. In order to further study the effect and mechanism of Sm2O3-APTES on the toughness of the composite material, the microscopic morphology of the tensile cross-section of the composite material was observed, as shown in Figure 10. When the Sm2O3-APTES content is 0-5 wt%, the fracture cross-section of the composites is neat and there is a single crack direction. Typical brittle fracture characteristics are shown in Figure 10a,b. When the content of Sm2O3-APTES is 5 wt%, a small amount is distributed in the AFG-90H matrix, which has little effect on the cracks and the good interface adhesion between Sm2O3-APTES and the matrix increases the material's toughness. When the content of Sm2O3-APTES is 10-15 wt%, the cracks are deflected, as shown by the yellow curve in Figure 10c,d, indicating that Sm2O3-APTES can One criterion for judging the toughness of polymer materials is the elongation at break. The elongation at break of the composite reached a peak of 0.68%, and T g was the lowest when the Sm 2 O 3 -APTES content was 15 wt% in this study. In order to further study the effect and mechanism of Sm 2 O 3 -APTES on the toughness of the composite material, the microscopic morphology of the tensile cross-section of the composite material was observed, as shown in Figure 10. When the Sm 2 O 3 -APTES content is 0-5 wt%, the fracture cross-section of the composites is neat and there is a single crack direction. Typical brittle fracture characteristics are shown in Figure 10a,b. When the content of Sm 2 O 3 -APTES is 5 wt%, a small amount is distributed in the AFG-90H matrix, which has little effect on the cracks and the good interface adhesion between Sm 2 O 3 -APTES and the matrix increases the material's toughness. When the content of Sm 2 O 3 -APTES is 10-15 wt%, the cracks are deflected, as shown by the yellow curve in Figure 10c,d, indicating that Sm 2 O 3 -APTES can be pinned in the matrix to prevent the cracks from propagating along the original direction among the contents [52]. The delamination section appears because the stress gradually concentrates as the content increases. When its content is 20-30 wt%, agglomerates began to appear, the delamination phenomenon caused by stress concentration is more obvious and the compatibility of the agglomerated part and the matrix becomes poor. Therefore, part of the agglomerate comes out of the AFG-90H matrix when the tensile stress is loaded, causing holes and generating surrounding micro-cracks [53]. The rapid propagation of micro-cracks reduces the toughness of the matrix. The mechanism was analyzed based on the SEM images, and its diagram is shown in Figure 11. At a low Sm 2 O 3 -APTES content (≤15 wt%), the crosslinking of particles and molecular chain can effectively disperse the stress. At a high Sm 2 O 3 -APTES content (>15 wt%), the tensile stress of the matrix to the fillers is higher than its interfacial adhesion force. Therefore, the agglomerations have interfacial debonding and holes [54,55]. A large amount of local plastic deformation is exhibited around the equator of the particles. Hence, the overall fracture toughness is reduced.
List of small modifications 5. Table 4 in Section 3 "Results and discussion" Please help to add "/" in the blank space of the third line in Table 4   be pinned in the matrix to prevent the cracks from propagating along the original direction among the contents [52]. The delamination section appears because the stress gradually concentrates as the content increases. When its content is 20-30 wt%, agglomerates began to appear, the delamination phenomenon caused by stress concentration is more obvious and the compatibility of the agglomerated part and the matrix becomes poor. Therefore, part of the agglomerate comes out of the AFG-90H matrix when the tensile stress is loaded, causing holes and generating surrounding micro-cracks [53]. The rapid propagation of micro-cracks reduces the toughness of the matrix. The mechanism was analyzed based on the SEM images, and its diagram is shown in Figure 11. At a low Sm2O3-APTES content (≤15 wt%), the crosslinking of particles and molecular chain can effectively disperse the stress. At a high Sm2O3-APTES content (>15 wt%), the tensile stress of the matrix to the fillers is higher than its interfacial adhesion force. Therefore, the agglomerations have interfacial debonding and holes [54,55]. A large amount of local plastic deformation is exhibited around the equator of the particles. Hence, the overall fracture toughness is reduced.

Neutronics Simulation
Monte Carlo methods are widely used in nuclear engineering [56,57]. In this work, the neutron shielding performance of Sm2O3/AFG-90H composite is simulated by the Su-perMC software. The one-dimensional physical model adopted for the neutron shielding performance of the shielding material is shown in Figure 12. The simulation was performed in fixed neutron source mode. An isotropic thermal neutron point source with an

Neutronics Simulation
Monte Carlo methods are widely used in nuclear engineering [56,57]. In this work, the neutron shielding performance of Sm 2 O 3 /AFG-90H composite is simulated by the SuperMC software. The one-dimensional physical model adopted for the neutron shielding performance of the shielding material is shown in Figure 12. The simulation was performed in fixed neutron source mode. An isotropic thermal neutron point source with an average energy of 0.0253 eV was used and the fluence was 10 6 n/(cm 2 ·s). The distance between the neutron source and the shielding material was 10 cm. The length perpendicular to the incident direction was defined as infinite, based on the assumption that all elements in the shielding material were uniformly dispersed. At least three measurements with analog point neutron detectors were performed. The neutron shielding performance of the material was statistically evaluated by neutron permeability I/I 0 . The following equation was used for I/I 0 [58]: where η is the transmittance, I and I 0 are the intensities of the transmitted and incident neutron fluences (n/cm 2 ), A is the accumulation factor, d is the thickness of shielding material (cm), Σ is the total macro-cross-section (cm −1 ) of all elements in the composite, and b is the background count. The secondary γ doses generated by the shielding material after capturing the thermal neutrons were recorded by the detector, and the flux was converted into dose through the dose conversion factor [59] to assess the level of gamma radiation. In addition, the Sm isotopic abundance and average thermal neutron absorption cross-section are listed in Table 3. Moreover, Table 4 summarizes the physical properties of epoxy-based neutron shielding materials, including the decomposition temperatures of different mass loss, tensile strength and neutron permeability of different epoxy-based composites [60][61][62]. It shows that Sm2O3-APTES/AFG-90H possessed better thermal stability and neutron shielding properties compared with other epoxy-based composites. The Sm2O3-APTES/AFG-90H with 20-25 wt% Sm2O3-APTES had good neutron shielding properties and the potential to be used as neutron shielding materials for advanced nuclear system.  The neutron shielding performance of Sm 2 O 3 -APTES/AFG-90H is shown in Figure 13. It is clear that the value of the transmittance η decreased linearly with the increase in shielding material thickness and decreased with the increase in Sm 2 O 3 -APTES content (as shown in Figure 13a), which indicated that the Sm element had good thermal neutron absorption performance. In the case of theoretical calculation, the η value for 30 wt% Sm 2 O 3 -APTES/AFG-90H with a thickness of 0.2 cm is 98.86%, which is higher than that of the AFG-90H matrix. The secondary γ dose released by the shielding materials after absorbing thermal neutrons is shown in Figure 13b. The secondary γ dose produced by the composites after absorbing thermal neutrons was less than 10 −14 Gy. With the increase in Sm 2 O 3 content, the trend of the γ dose gradually transforms from a linear increase to a logarithmic increase. This indicates that the secondary γ dose generated by the composite material absorbing thermal neutrons will reach a saturation value with the increasing content of Sm 2 O 3 . The secondary γ at this dose level will be scattered in the environment without causing adverse damage to the human body or the environment. Moreover, Table 4 summarizes the physical properties of epoxy-based neutron shielding materials, including the decomposition temperatures of different mass loss, tensile strength and neutron permeability of different epoxy-based composites [60][61][62]. It shows that Sm2O3-APTES/AFG-90H possessed better thermal stability and neutron shielding properties compared with other epoxy-based composites. The Sm2O3-APTES/AFG-90H with 20-25 wt% Sm2O3-APTES had good neutron shielding properties and the potential to be used as neutron shielding materials for advanced nuclear system. Moreover, Table 4 summarizes the physical properties of epoxy-based neutron shielding materials, including the decomposition temperatures of different mass loss, tensile strength and neutron permeability of different epoxy-based composites [60][61][62]. It shows that Sm 2 O 3 -APTES/AFG-90H possessed better thermal stability and neutron shielding properties compared with other epoxy-based composites. The Sm 2 O 3 -APTES/AFG-90H with 20-25 wt% Sm 2 O 3 -APTES had good neutron shielding properties and the potential to be used as neutron shielding materials for advanced nuclear system.

Conclusions
In this paper, a new type of high-performance epoxy resin AFG-90H and rare earth oxide Sm 2 O 3 composites was prepared. The effects of Sm 2 O 3 -APTES content on thermal properties, mechanical properties and neutron shielding properties were studied. The mechanism of its effect on mechanical properties was deeply studied. The main conclusions are as follows: (1) The uniform dispersion of Sm 2 O 3 particles and good interface compatibility improved the strength and stiffness of Sm 2 O 3 -APTES/AFG-90H composites. When the Sm 2 O 3 -APTES content is 15 wt%, the composites have higher fracture toughness. (2) The Sm 2 O 3 -APTES hindered the thermal motion of molecular chains and improved the thermal stability of the composite at 340-380 • C. Simultaneously, the glass transition temperature T g of the composites is slightly increased. (3) The shielding simulation showed that 30 wt% Sm 2 O 3 -APTES/AFG-90H had a higher neutron shielding performance than the AFG-90H matrix. After capturing neutrons, the secondary γ dose was 8.5 × 10 −15 Gy. (4) The appropriate range of Sm 2 O 3 -APTES content in the epoxy matrix is 20-25 wt% to obtain higher properties.
In future work, the properties of Sm 2 O 3 -APTES/AFG-90H under neutron irradiation will be studied. The results indicate that the polymer composites have the potential for application in radiation protection.